The present invention relates, in general, to integrated circuits and, more particularly, to power converter integrated circuits.
Power supplies typically employ rectifier circuits and a power transformer for generating Direct Current (DC) voltage from an Alternating Current (AC) voltage source. Usually, a first rectifier circuit is connected across a primary winding of the power transformer. The first rectifier circuit receives the AC voltage and provides a rectified voltage to the primary winding. A second rectifier circuit is connected across a secondary winding of the power transformer and provides a DC output voltage. The DC output voltage can be used for supplying operating power to electronic applications such as computers, televisions, printers, battery chargers, etc.
Generally, a feedback circuit is coupled to the power supply in order to regulate the DC output voltage. The feedback circuit includes a power switch connected to the primary winding and a pulse width modulator for operating the power switch. The pulse width modulator sends a gate signal to the power switch to turn the power switch on and off. When the switch is turned on and off, the energy stored in the primary winding is modulated which controls the DC output voltage.
The DC output voltage is used to change the duty cycle of the gate signal. Thus, the DC output voltage provides feedback to the power switch for regulating the DC output voltage to a desired DC voltage level.
One problem with switching regulated power supplies is the potential failure of the power switch when the power supply is initially powered up. During initial power-up or start-up, a capacitive load at the output of the power supply is charged to some desired voltage level from an initial discharged state. Typically, the power switch is turned on for a large percentage of the time in order to increase the DC output voltage. In other words, the gate signal is at a maximum duty cycle where the widths of the pulses are relatively long and the power switch stays on longer during each cycle of the gate signal. This may cause the power transformer to saturate and the power switch to be electrically or thermally stressed resulting in the failure of the power switch during start-up.
Accordingly, it would be advantageous to have a power supply and method for operating the power supply that prevents failure of the power switch during initial start-up. It would be of further advantage to have a power supply and method that reduces the amount of time that the power switch is on during initial start-up. In addition, it would be advantageous for the power supply and method to improve the overall efficiency of the power supply.